Replacing cement mortar in old stone walls is one of those jobs where the wrong approach can cause more harm than good. Cement pointing was often used in the twentieth century as a quick fix, but it has proven disastrous for historic masonry. The good news is that with patience and the right materials, you can restore a wall to its original breathable, flexible state.
Cement mortar is hard, rigid and largely impermeable. Old stone walls are designed to move slightly with temperature and moisture changes, and they rely on soft, porous lime mortar to accommodate that movement. When cement is used, it traps moisture inside the wall. That moisture has nowhere to go, so it forces its way out through the stone itself. Over time, this causes spalling, where the face of the stone flakes away, and can lead to frost damage and structural decay. Cement lacks flexibility, so as the wall shifts, cracks appear and water penetration worsens. Lime mortar does the opposite: it is sacrificial, meaning it decays first and can be replaced without harming the masonry.
Before you pick up a chisel, take time to understand what you are dealing with. Not every cement joint needs to come out immediately. If the cement is sound, well-bonded and not causing damp problems, it may be best left alone. But if you can see damp patches, crumbling stone, or wide cracks, removal is likely necessary. Check the following:
If the wall is listed or in a conservation area, you may need consent before carrying out work. Check with your local conservation officer.
Removing hard cement is delicate work. Power tools can be tempting, but angle grinders and mortar rakes generate a lot of dust and can easily slip, gouging the stone. If you must use a grinder, fit a diamond blade and a dust extraction unit, and work with extreme care. For most historic walls, hand tools are safer and give you far more control.
Use a plugging chisel and a lump hammer to break out the cement in small sections. Work from the centre of the joint outwards, and never lever against the stone edge. A cold chisel can be used for stubborn areas, but keep it sharp and angled away from the stone face. A mortar rake or a narrow pointing trowel can help clear debris. Wear gloves, goggles and an FFP3 dust mask, especially if the mortar is old and potentially contains lead or silica. Work in small areas, vacuum or brush out dust as you go, and stop if you feel the stone starting to move.
Lime mortar comes in two main families: non-hydraulic (air lime) and hydraulic (natural hydraulic lime, or NHL). Non-hydraulic lime putty is the traditional choice for most historic stone. It sets by carbonation, is highly breathable and very flexible. It is ideal for soft, porous stones and sheltered walls. Hydraulic lime sets more quickly and is more water-resistant, making it suitable for exposed locations or damp conditions. NHL 2 is the weakest and most breathable; NHL 3.5 is a good all-rounder; NHL 5 is too strong for most old stone. As a rule, choose the weakest lime that will do the job.
Mix your lime with a well-graded sharp sand. Avoid building sand, which often contains clay and salts. A typical ratio for lime putty is 1 part putty to 2.5 or 3 parts sand by volume. For NHL, use 1 part lime to 2.5 or 3 parts sand. The mortar should be slightly weaker than the stone. If in doubt, test a small area first.
Lime mortar improves with age. If using lime putty, mix it with sand at least a day before use and cover it. For NHL, mix dry, then add water slowly until you reach a firm, buttery consistency. Dampen the joints with water before applying, so the lime does not dry out too quickly. Pack the mortar into the joint in layers if it is deeper than 15mm. Use a pointing trowel or a finger to push it in, then finish with a wooden or metal tool. Avoid smoothing the surface too much; a slightly rough finish helps moisture evaporate. When the mortar is firm but still workable, brush it back with a churn brush to expose the aggregate and close the surface.
Lime mortar needs time and moisture to cure. It does not dry like cement; it carbonates. Keep the new pointing damp for at least three to seven days. Mist it with water, or cover it with damp hessian. Protect it from direct sun, strong wind and frost. In hot weather, mist more often. In cold weather, wait for milder conditions. Do not paint or seal the mortar. It must remain breathable. After a few weeks, the lime will harden and blend with the old stone. Any small shrinkage cracks can be filled with more lime mortar. With proper care, your repointing will last for decades and allow the wall to breathe as it was meant to.
Lime & Stone Journal shares practical, down-to-earth guidance on lime plastering and historic building restoration for readers across the UK.
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